In situ generation of Cu- and Ag–Sn alloys from metal sulfides for CO2 reduction†

Ag, Cu and Sn based electrocatalysts promise high CO₂ reduction kinetics and efficiencies on gas diffusion electrodes. Ag, Cu, Sn sulfide catalysts in particular may offer altered electronic properties and product selectivity, while still being easy to manufacture in scaleable synthesis routes. Comparing the CO₂ reduction (CO₂RR) performance of Cu₃SnS₄, Ag₃SnS₄, Cu₂S, SnS and Ag₈SnS₆ at 100 mA cm⁻², formate is found to be the primary CO₂RR product with a faradaic efficiency of 57% for Cu₃SnS₄ and 81% for Ag₃SnS₄. Characterization by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction revealed the formation of Ag₃Sn and Cu₃Sn alloys from the corresponding sulfide species during CO₂RR. But while the Cu₃Sn based electrode surface decomposed into CuO and SnO after 2 h at −100 mA cm⁻², metallic Ag₃Sn sites on the corresponding electrode surface could be detected by XPS after removing the surface layer. Using density functional theory, the binding energies of *H, *CO and *OCHO on Cu₃Sn and Ag₃Sn were computed to identify possible catalytic sites. Thereby, Sn was found to render both Cu and Ag highly oxophilic resulting in strong adsorption of carboxylic functionalities, enabling formate production with a partial current density of up to 162 mA cm⁻².